Chapter 27:

But everything was still all right, they’d tell Dad someday, and meanwhile...

...meanwhile Dumbledore had happened to sneeze while passing them in the hallway, and a small package had accidentally dropped out of his pockets, and inside had been two matched wardbreaker’s monocles of incredible quality. The Weasley twins had tested their new monocles on the “forbidden” third-floor corridor, making a quick trip to the magic mirror and back, and they hadn’t been able to see all the detection webs clearly, but the monocles had shown a lot more than they’d seen the first time.

Of course they would have to be very careful never to get caught with the monocles in their possession, or they would end up in the Headmaster’s office getting a stern lecture and maybe even threats of expulsion.

It was good to know that not everyone who got Sorted into Gryffindor grew up to be Professor McGonagall.

I think it's been mentioned a few times, but I can't remember a specific citation off the top of my head.